Method and system for television communication



June 39- T. T. GOLDSMITH. JR 76 METHOD AND SYSTEM FOB TELEVISIONCOMMUNICATION Fiied 01112, 1938 s Sheets-Sheet 3 INVENTOR v monks r.aoLnsun'n. JR.

Patented it... 27, 1939 METHOD AND SYSTEM FOR TELEVISION COMMUNICATIONThomas T. Goldsmith, Jr., Montclair, N. J., assignor to Allen B. Du MontLaboratories, Inc., Upper Montclair, N. J., a corporation of DelawareApplication March 12, 1938, Serial No. 195,441

2 Claims.

My invention relates to improvements in methods and systems fortelevision communication.

In the more successful methods and systems proposed heretofore fortelevision communica- 5 tion, and employing a cathode-ray pick-up tubeat'the transmitter and a cathode-ray viewing tube at the receiver, ithas been required that I there be at each station two circuits fordeflecting the scanning ray at the line and field frequencies,respectively. Also, it has been required that line and field pulses betransmitted on the same carrier wave as the video-signals, and thesepulses have been used at the receiver for synchronization. In theseprior methods and systems, 16' not only has difficulty been encounteredin holding the deflecting circuits at the receiver locked in step withthose at the transmitter, but a sub stantial portion of the transmittedenergy has been expended in transmitting the synchronizing 20 pulses.

With the foregoingin mind, it is one of the objects of my invention toprovide an improved 'method and system for television communicationemploying cathode-ray tubes at both stations,

and in which no synchronizing pulses are required to be transmitted tothe receiver, and in which either the deflection circuit at the linefrequency or both deflection circuits at the line and field frequencies,respectively, can be omitted at the receiver.

Another object of my invention resides in the provision of an improvedmethod and system for television communication which has advantages overthose proposed heretofore in the way of greater simplicity ofconstruction'and manner of operation, and higher efiiciency.

Another object of my invention resides inthe provision of an improvedmethod and system for television communication by which the various 40necessary signals can be transmitted by wireless over substantiallygreater distances than it has been possible with the various methods andsystems proposed heretofore, the possible range of wireless transmissionfrom the transmitting station directly to a receiving station being ofthe order ofhundreds of miles.

.-,,j-0th6l' objects and advantages will hereinafter appear. a 1 j\Irnaccordance with my invention, voltage waves [5o atthetrespectiveline and field frequencies are g eneratedat the transmitter andtransmitted by rvradiolto' the-receiver. These same waves are apdtoicause or at least govern scanning action is ning-action of theviewing tube at the receiver,

Eof t epick-up tube at the transmitter and scanso that the electron rayin the tubes must of necessity be always in step during the scanningaction. Because of the impossibility of this system getting out ofsynchronization as to both line and frame or field frequencies, a muchhigh- 5 er interlace ratio is possible. This allows a given detailpicture to be transmitted over a much narrower frequency band. Likewise,the transmission of much higher definition pictures over the samefrequency band can also be accom- 10 plished.

Another advantage of this system is that it will receive pictures oftransmitters operating at any desired line and frame frequencies. Withthe present systems, it is necessary that all trans- 5 mitters operatewith exactly the same respective scanning frequencies if all receiversare to be capable of receiving from any one of the transmitters.

My invention resides in the improved meth g0 and system of the characterhereinafter described and claimed.

For the purpose of illustrating my invention, an embodiment thereof isshown in the drawings, wherein 25 Figures 1 and 2 are simplified,diagrammatic views of a television transmitting station and a televisionreceiving station, respectively, constructed and operating in accordancewith my invention; 30

Figs. 3 and 4 are graphical representations of electrical wave formspossible for use in my improved system for governing the scanningaction; and

Figs. 5, 6 and '7 show circuit details for use in 35 my improved system.

In Fig, 1, the reference numeral I0 designates a cathode-ray pick-uptube of a conventional construction, and comprises a mosaic,photoelectric screen on which a light image of the object 4 isprojected, an electron gun for generating a ray of electrons directed atthe screen, and two sets of deflecting plates for deflecting theelectron ray at the line and field frequencies respectively so that'itis caused to scan the screen. The picture 45 I or video signals arethereby developed, and fed by an output connection II to a modulatingamplifier l2.

A first, main carrier wave is provided by an oscillator l3. In the poweramplifier stage I! of the oscillator, this carrier wave is modulatedbythe wide frequency band video signal through the modulating amplifierI2. The signals from the amplifier l4 are supplied by a connection I5 toa mixer circuit l6. I

The reference numerals I1 and i8 designate generators of voltage wavesof sine form for the sweeps and which are, respectively, at the desiredfield and line frequencies. The low-frequency sweep, by means of theamplifier l9, modulates the sub-carrier oscillator in its amplifierstage 2|. The high-frequency sweep, by means of the amplifier 22,modulates the sub-carrier oscillator 23 in its amplifier stage 24.

The audio or sound signals from the microphone 25 are passed through anamplifier 28.

The two sweep signals and the audio signals are applied through a mixercircuit 21 to modulate, in the power amplifier stage-28 of an oscillator29, a second, main carrier wave: The signals from the amplifier 28 aresupplied by a connection 30 to the mixer circuit IS.

The two carrier waves, one modulated by the video signals and the othermodulated by the two sweep signals and the sound signals, are mixed inthe mixer circuit l6 and fed to a common antenna 3| for radiotransmission.

Although the two sweep signals are transmitted in the form of sinewaves, in Fig. 1 they are not used as such for direct application to thetwo respective sets of electrostatic deflecting plates, but are firstchanged or modified to waves of saw-tooth form by wave-form modifyingmeans or networks 32 and 33. An example of such a network is shown inFig. 5, in which the sine-wave input is fed through a rectifier 34 to afilter section consisting of a condenser 35 and a resistance 36, fromwhich section is delivered the desired output wave of saw-tooth form.

In Fig. 1 it is proposed to use a frequency-multiplying method for thepurpose of maintaining constant, particularly where interlaced scanningis used, the relation of line frequency to field frequency. A system orcircuit for such purpose is shown in Fig. 6, in which a low-frequencyvoltage wave of sine form, such as the 60-cycle power main frequency, isfed through successive fullwave rectifiers 31, 38, etc., followed byfilters, as shown, to allow passage of the double-frequency harmonics.In this way there is produced a final and much higher frequency whichcan be utilized for the high frequency sweep coordinated with theoriginal low-frequency sweep for the two respective deflections forscanning. In Fig. 6 there is shown only two stages of thefrequency-multiplication scheme. A transformer primary 3!] is excited by60 cycles, and the tapped secondary supplies the full-wave rectifier 31which de-' livers 120 cycles to the tuned filter comprising thecondenser 4| and the next transformer primary 42. The 120-cycle signalis supplied by the tapped secondary 43 to the full-wave rectifier 38 andthence at the double frequency of 240 cycles to the condenser 44 and thenext succeeding transformer primary 45. The tapped secondary 46 deliverssignals to the next stage, and so on until the required high frequencyfor linescanning is'obtained. In event the resistances of thetransformers are too low, resistances may be inserted in the rectifiercircuits for the purpose of limiting the direct current.

An alternative and simplified system or circult, for the same purpose asthat shown in Fig. 6, is shown in Fig. 7. In Fig. 7, the referencenumerals 41, 48 and 48 designate transformer windings, connected asshownwith respect to full- :gave rectifiers 50 and 5| and condensers 52and In Figs. 6 and '7, conventional amplifier stages may be employedwhen needed. Also, if it is desired to produce a phase relation of thefinal frequency, phase-shifting networks may be inserted in one or morestages of the frequencymultiplication system. In cases where it'isdesired, as in interlaced scanning, that the final, high frequency be anodd multiple of the original low frequency, rather than an even multipleof the latter, it is proposed to use an excess number of doubler stagesand then to employ one or more conventional multivibratorfrequency-reduction stages to obtain the desired odd-scanning frequencyfor the line sweep.

In'Fig. 1, the video signals, instead of being fed to the mixer l6, maybe applied by the connection l5 to another antenna system independent ofthe antenna 3|.

At the receiving station, shown in Fig. 2, the antenna 54 applies thecombined carrier signals, from the transmitter antenna 3|, to a broadradio-frequency amplifier 55. 'A local oscillator 56 reacts with thesesignals in a first detector stage 51, on the superheterodyne principle,to produce two intermediate-frequency signals which are supplied to therespective intermediate-frequency channels or stages 58 and 59.

After suitable amplification, the video signal is detected at 60 andapplied by a connection 5| to a scanning device 62. The device 62 isrepresented as being in the form of a cathode-ray tube of a commonconstruction, and comprising a fluorescent screen, an electron gun fordeveloping a ray of electrons directed at the screen, and two sets ofelectrostatic plates for deflecting the electron ray at the respectiveline and field frequencies to cause it to scan the screen. The videosignals are applied to a control electrode of the electron gun wherebythe intensity of the electron ray is made to vary with the picture orvideo signals.

The signal in the intermediate-frequency stage 59 still contains the twosweep signals and the audio signals, which now require separation. Asecond detector 83 of relatively wide band output, removes the compositesignals from the intermediate-frequency carrier, the output from thisdetector going to three selective filters 64, and 85. The low-frequencysweep signal," still in the form of modulation upon its carrier, isdetected at 61 to produce in the output line 68 a voltage wave of sineform which is changed, by a waveform modifying network 69 similar to thenetwork 32 in Fig. 1, to a saw-tooth voltage wave which is appliedacross the corresponding deflecting plates. Likewise, the high-frequencysweep signal, still in the form of modulation upon its carrier, isdetected at 10 to produce in the output line H a voltage wave of sineform which is changed by a wave-form modifying network 12 similar to thenetwork 33 in Fig. 1, to a saw-tooth voltage wave which is appliedacross the corresponding deflecting plates. I

The audio signal, in its original form from the filter 66, is fed to aloudspeaker I3.

In Fig. 2, the unit v85 represents an automatic signal-level controlcircuit which, in the well known manner, utilizes a rectified portion ofthe received signal to adjust the bias on previous stages to aid inmaintaining substantially constant signal output. In the present case,signal for the unit 85 is taken from the high-frequency sweep circuit.Automatic control, based on the fixed, high-frequency sweep signal, willbe steady and independent of background variations of the video signal.As shown, the output control signal from the unit 35 is applied tocontrol bias in the first detector stage 51 and in theintermediatefrequency channel 59.

Instead of transmitting-sinusoidal sweeps as in Fig. 1, the generatorsl1 and I8 may be made to develop saw-tooth sweep wave forms, as shown inFig. 3. Such a sweep wave form is characterized by having a relativelywide frequency band necessary for faithful transmission. Its fundamentalfrequency and also its harmonics up to about the tenth should beprovided for. This type of scanning provides systematic and uniformcoverage of the screen, and though high in harmonic content can still beused with this system by proper design of the intermediate-frequencyfilters.

Another wave shape, illustrated in Fig. 4, may be transmitted, in whichcase the generators l1 and I8 are designed accordingly. It ischaracterized by equal slopes of the forward and return traces, thuscontaining a much lower harmonic content, simplifying the transmissionproblem considerably. Interlace here takes on a somewhat differentinterpretation, but pictures produced in this manner prove to be verysatisfactory.

-cies indicated in Figs. 1 and 2 are by way of example only, and thatthese might vary widely to meet particular requirements.

In case the other shapes of scanning signa are desired, it will benecessary to choose other frequency bands than here shown, but thegeneral principles will be the same and are believed to have beensufficiently disclosed for those skilled in the art. v

A feature of my improved system of television is the exclusivegeneration of the sweeps at the transmitter and actual transmission ofthese sweeps to the receiver, thus insuring perfect synchronism ofscanning. No synchronization is required. The interlace can bequite'complex. with a high interlace ratio such as 4, for example, andyet will remain in adjustment. Receiver controls are greatly simplified.

Another feature of my improved system is the use of sweeps of simplewave form, thereby providing a narrow frequency band and consequentsimplified amplification and transmission of the sweeps.

Still another feature of my improved system resides in the reducedvideo-frequency band possible with the multiple interlace or highinterlace ratio, yet giving high definition pictures. With this reducedvideo-band width and its relatively narrow associate band containing theaudio and the sweeps, it is practical to utilize other and lower carrierfrequencies than those suggested in Figs. 1 and 2, and thus to employcarriers which are not limited to an optical horizon for satisfactorycoverage.

The frequency width of the video signals illustrated in Figs. 1 and 2has been shownas those characteristic of a picture of about 600- linedefinition and the carriers chosen are merely suggestive. One can seethat even with this provision for a 'video channel of higher definitionthan provided in a unit with the 441-line standards with 2 to 1interlace, the ultra-high frequency band still is not increased overthat required for the 441-line 2 to 1 interlace pictures. Now if only441 lines are desired, using the system herein disclosed of multipleinterlace, and by very stringent selectivity in closely spaced channels,it is possible to reduce the channel band width to approximately 1megacycle each side of the ultra-high frequency carrier, thus makingpractical the utilization of carriers in the range of 30 megacycles,with their consequent potential coverage over large areas and to greatdistances. This is a very marked advantage of the present improvedmethod and system disclosed herein.

Another advantage of my improved system is that a receiver of the typeillustrated would-be versatile in its ability to accept transmissionsfrom different stations utilizing different degrees of detail. Given aspecified band pass in each channel, then pictures of any detail up to acertain limit could be accepted by simply tuning to the appropriatqR. F.carrier of the station in question, without necessity of localadjustment to duplicate their particular sweep system. It willfacilitate public field installation of television systems without thefear of equipment becoming immediately obsolete when definition changesare "desired.

Another advantage of my improved system is the possibility of the use ofautomatic volume control, or automatic signal-level control byapplication from one or the other fixed signals in the sweep circuits.Automatic control based on this signal will be steady and independent ofbackground variations of the video signal.

It has been found that excellent pictures may be obtained when integralratios between the scanning frequencies are purposely avoided. Thetelevision system herein disclosed allows practic'ally any type ofinterlacing to be employed. For example, assume that there is employed avertical sweep of sixty fields per second, which is well beyond theflicker limit. Now a horizontal frequency of 3000 cycles per secondwould give fifty lines with exact progressive scanning. Now an increaseto 3030 cycles will provide 101 lines in the complete picture with a twoto one interlace ratio. The picture repetition rate is reduced to 30 persecond, but there is still no appreciable loss in picture continuity.Now use some horizontal sweep such as 3002 cycles per second and thescanning will return to a given configuration only after half a second.This produces an effect of lines drifting by when the pattern is greatlymagnified, but on the conventional viewing screen, the use of thisscheme employing other than integral ratios between sweep frequencies,makes the line structure practically indiscernible. Use of this peculiarratio of sweeps is practical with the herein described televisionsystem, as it is necessary to regulate the sweeps.

only at the transmitter.

A feature of my improved system resides in the utilization of actualscanning wave shape transmission suitable for electrostatic cathode-raydeflection, though with proper amplifiers, the use of electromagneticdeflection is also possible. It is obvious that utilization ofelectrostatic deflection at the receiver would allow the same to followthe changes in scanning frequency or wave-form, when these might bevaried at the transmitter. It would be more difficult' to designanelectromagnetic system having this same flexibility.

Another feature of my improved system is the possibility of furthersimplification of the receiver equipment if operated on a common powermain with the transmitter, for then the power mains frequency may beemployed for the low sweep.

reducing the complexity of the receiver in that locality. However, thetransmitter carrier may still contain this low sweep component even whenexciting such a receiver, for no disturbance will be experienced ifunits 64 and 61 in Fig. 2 are omitted and replaced by suitableattachments to the power mains. In the light of this modiflcation, itwould be most practical to secure the automatic signal-level controlvoltage from the high sweep channel which will be present in allreceivers.

Still another advantage of my improved system is that its receivers willbe versatile enough to receive both the highest definition pictures andthe low definition pictures even of the amateur experimenters where costof transmitter equipment is'a limit of the definition which can beprovided.

It will be understood that various modifications are possible withoutdeparting from the spirit of my invention or the scope of the claims.

I claim as my invention:

1. In the art of television communication wherein it is required thatelectrical sweep waves of low frequency and high frequency respectivelybe supplied to the receiver for governing scanning action thereat, thesteps in the method of operation which consist in modulating a firstmain ultra-high frequency carrier with the video signal, modulating asub-carrier with the highfrequency sweep wave, modulating a second mainultra-high frequency carrier with the modulated sub-carrier, and feedingsaid modulated main carriers to radio transmission means.

2. In the art of television communication wherein it is required thatelectrical sweep waves of low frequency and high frequency respectivelybe supplied to the receiver for governing the scanning action thereat,the steps in the method of operation which consist in modulating a firstmain ultra-high frequency carrier with the video signal, modulating asub-carrier with the lowfrequency sweep wave, modulating a differentsub-carrier with the high-frequency sweep wave, mixing the two modulatedsub-carriers with audio signal, modulating a second main ultra-highfrequency carrier with the composite signal comprising the two sweepsand the audio signal, and feeding said modulated main carriers to radiotransmission means.

THOMAS T. GOLDSMITH, JR.

